The invention relates to a calibration system for the calibration of mass flow controllers of an apparatus used for a chemical vapor phase growth.
The techniques of chemical vapor phase growth are used generally in semi-conductor industries and in connection with the production of optic fibers (MCVD modified chemical vapor deposition-technique, OVPO outside vapor phase oxidation-technique, VAD vapor phase axial deposition-method and the like). Apparatuses applying said methods comprise vapor channels intended for different materials (such as SiCl.sub.4, GeCl.sub.4, POCl.sub.3, BBr.sub.3) and including mass flow controllers.
A conventional apparatus for controlling a vapor flow and for the transportation thereof into a system to be used is illustrated in FIG. 1. Therein a carrier gas, e.g., oxygen, is passed through a stop valve C to a mass flow controller M. This passes the gas flow into a bubbler B, i.e. inside a liquid F contained therein.
The empty upper portion of the bubbler B is filled with a saturated (or at least by near saturated) vapor of the used liquid F. The carrier gas carries the vapor into the using system. In an ideal case, the vapor flow rate Q (moles/min) is ##EQU1## wherein Pi is the vapor pressure of the liquid at the used temperature (T), P is the total bubbler pressure, R a general gas constant, T the absolute temperature, K a factor dependent on the coefficient of efficiency of the vaporization and V a carrier gas flow rate (standard 1/min). So the amount of vapor can be regulated by controlling the carrier gas flow rate V.
This in turn can be effected by means of a thermal mass flow controller M wherein the gas flow rate in an ideal case is directly proportional to the control voltage. (Such devices are commercially available, manuf. Tylan and Brooks, e.g.). A conventional characteristic of a mass flow controller (gas flow rate as a function of the control voltage) is shown in FIG. 2, in which a curve a shows an ideal characteristic and a curve b an actual characteristic.
A vapor control system of the type described is more advisable than a flow controller based on direct measuring of the amount of vapor (a so called source controller, e.g., Source V, manuf. Tylan). The thermal conductivity of the carrier gas and the vapor is therein measured by means of a special measuring element which comprises parts which are usually made of stainless steel. However, the chemicals used in the production of optic fibers, e.g., cause corrosion in the metal parts and dissolved metals may deteriorate the quality of the fiber. Corrosion also brings about a change in the characteristic of the apparatus, whereby the controller does not operate properly. The controller also requires frequent cleaning and must be calibrated thereafter. The above-mentioned disadvantages are referred to also in U.S. Pat. No. 4,436,674.
In FIG. 2, the curve b shows the characteristic of a thermal mass flow controller. As to the shortcomings, there is, among others, the offset value o (typically approx. 2%). Also the fullscale flow fs (e.g. 1000 ml/min) may deviate from the normal value in commercially available controllers. In optic fiber production, a wide linear range of control as well as an accurate control over the whole range are of importance.
As illustrated in FIG. 2, commercially available mass flow controllers are not ideal, which may lead, e.g., in a situation illustrated in FIG. 3, wherein the upper and lower limiting values of the controllers 1 and 2 deviate from each other, causing discontinuity in the control process. On the other hand, the operation of the controllers is not completely linear, either. In addition, replacing with a new one or cleaning of some of the controllers may cause rather a wide variation in the characteristic.
Besides, practice has shown that already very small deviations in the operation of the controllers may result in highly disadvantageous changes in such properties of e.g. the preform of a MCVD optic fiber as an index of refraction profile, a numeric aperture, geometry, etc. These deviations, maybe minor as such, can, however, have an extremely great effect on the critical properties of the fiber, particularly on the band width, attenuation and geometry. Consequently, not even the deviation .+-.1% guaranteed for the controller on the maximum limit can always be regarded as acceptable.